Among various aqueous solutions of chromium salts, aqueous solutions of chromium nitrate are known to be used as surface treatment agents for metals. Aqueous solutions of chromium phosphate are used as treatment solutions for forming chemical conversion coatings on zinc-plated surfaces of various metals, such as iron, nickel, and copper (refer to Patent Document 1).
In a known method for producing a chromium salt, such as chromium nitrate, chromium phosphate, or chromium chloride, ore is subjected to alkali oxidizing roasting to prepare a sodium dichromate solution, sulfuric acid is added to the sodium dichromate solution, which is then subsequently reduced with an organic substance to produce a chromium sulfate solution, caustic soda or soda ash is added thereto to form a precipitate of chromium hydroxide or chromium carbonate, which is subsequently subjected to filtration and water washing, and then nitric acid, phosphoric acid, or hydrochloric acid is added thereto for dissolution. In another known method, chromium ore is reduced in an electric furnace using a carbon reductant to produce high-carbon ferrochromium, the high-carbon ferrochromium is extracted with sulfuric acid, the resulting solution is electrolyzed to obtain metallic chromium, and nitric acid or hydrochloric acid is added to the metallic chromium to produce chromium nitrate or chromium chloride.
Furthermore, with respect to chromium nitrate, a production method is also known, which includes the steps of preparing a mixed solution of chromium trioxide and nitric acid by mixing nitric acid with chromium trioxide in a stoichiometric amount required to produce chromium nitrate or more, and adding an excessive amount of an organic reducing agent composed of an alcohol, an aldehyde, or a carboxylic acid, which is derived from a carbohydrate selected from monosaccharides, disaccharides, and starches, or a mixture of these, to the mixed solution of chromium trioxide and nitric acid prepared in the previous step to reduce chromium trioxide, thereby producing chromium nitrate (for example, refer to Patent Document 2).
With respect to chromium chloride, the present applicant has proposed a method in which an aqueous solution of chromic acid is allowed to react with an organic reducing agent capable of substantially completely decomposing into carbon dioxide gas and water by reaction with hydrochloric acid and chromic acid, thereby producing a high-purity aqueous solution of chromium chloride (for example, refer to Patent Document 3).
Among known methods, in the method in which chromium hydroxide is dissolved in nitric acid, it is very difficult to perform water washing of the precipitate of chromium hydroxide produced by adding caustic soda or soda ash to chromium sulfate, and it is not possible to remove impurities, such as sodium and sulfate salts, in chromium hydroxide. This problem remains to be solved.
In the method according to Patent Document 2, for example, mixing of nitric acid with chromium trioxide in a stoichiometric amount required to produce chromium nitrate or more is effective in decreasing the amount of hexavalent chromium. However, depending on the conditions, the added reducing agent may react with nitric acid instead of chromium trioxide, resulting in generation of NOx. Consequently, not only a de-NOx system may be required, but also sudden reaction may cause dangerous conditions.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-268562
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-339082
Patent Document 3: Japanese Unexamined Patent Application Publication No. 1-176227